The operation of vehicles such as those used for plowing or scraping snow and/or ice from roads, airport runways and similar surfaces and for spreading traction enhancing materials such as sand and/or salt requires the installation of a hydraulic system that supplies power to operate the various components of spreader and plow equipment. The usual installation includes a single gear pump that pushes hydraulic fluid through an open center valve with a power beyond connection, which is used to operate the plow functions. The power beyond is connected to the pressure of the spreader valve, from where it returns to a hydraulic fluid tank or reservoir, or is partially routed to the spreader's hydraulic motors. The principal problem of this circuit is the stoppage of the spreader when any plow function is operated. Various solutions have been used to remedy this problem.
Tandem pumps may be used in two completely separate hydraulic circuits. One pump supplies the plow hydraulic functions, the other one supplies the spreader only. Simultaneous operation is rendered possible for both the plow and the spreader, but several major inconveniences remain. The system is more expensive, more complicated, requires a larger hydraulic fluid reservoir and consumes more energy.
Another solution is the use of a variable displacement piston pump that is usually controlled by sensing the load. The load sensing pump works in conjunction with closed center valves that share a common pressure supply, which is the pump. This constitutes a normal load sensing circuit, with all its usual benefits. The main problem associated with this solution is the very high cost of load sensing valves and pumps.
Therefore, there is a need for a cost effective load sensing hydraulic system capable of providing hydraulic fluid to operate the various components of plow/spreader type equipment.